Many device applications require control of the grain boundary character of polycrystalline materials which form part of the device. For example, in high temperature superconductors (HTS) grain boundary characteristics are important. The significant effect of grain boundary characteristics on current transmission across superconductor grain boundaries has been clearly demonstrated for YBa2Cu3O7x (Y123). For clean, stoichiometric boundaries, the grain boundary critical current Jc (gb) appears to be determined primarily by the grain boundary misorientation. The dependence of Jc (gb) on misorientation angle has been determined by Dimos et al. [1] in Y123 for several grain boundary types, which can be formed in epitaxial films on bicrystal substrates. These include [001] tilt, [100] tilt, and [100] twist boundaries [1]. In each case, high angle grain boundaries were found to be weak-linked. The Jc value decreases exponentially with increasing grain boundary misorientation angle in artificially fabricated bicrystals of YBCO films [1]. The low Jc observed in randomly oriented polycrystalline Y123 can be explained by the small percentage of low angle boundaries, the high angle grain boundaries impeding long-range current flow.
Recently, the Dimos experiment has been extended to artificially fabricated [001] tilt bicrystals in Tl2Ba2CaCu2O8 [2], Tl2Ba2Ca2Cu3Ox [3], TlBa2Ca2Cu2Ox [4] and Nd1 85Ce0.15CuO4 [3]. In each case it was found that, as in Y123, Jc depends strongly on the distribution of grain boundary misorientation angles. Although no such measurements have yet been made on Bi-2223, data on current transmission across artificially fabricated grain boundaries in Bi-2212 indicates that most large [001] tilt [3] and twist [5,6] grain boundaries are weak links, with the exception of some coincident site lattice (CSL) related boundaries [5,6]. It is likely that the variation in Jc with grain boundary misorientation in Bi-2212 and Bi-2223 will be similar to that observed in the well characterized cases of Y123 and Tl-based superconductors. Hence in order to fabricate high temperature superconductors with very high critical current densities, it is necessary to biaxially align the grains to produce a high percentage of low angle grain boundaries. This has been shown to result in significant improvement in the superconducting properties of YBCO films [7–10].
A simple method to fabricate long lengths of textured substrates with primarily low-angle grain boundaries for epitaxial deposition of high temperature superconducting (HTS) materials is disclosed by Goyal et al. [10]. This method is known as Rolling-Assisted-Biaxially-Textured-Substrates (RABiTS). Four U.S. patents have been issued on this process and related process variants (U.S. Pat. Nos. 5,739,086, 5,741,377, 5,898,020, 5,958,599 and 5,944,966). In the RABiTS method, the substrate formed is a polycrystalline substrate having primarily low angle grain boundaries.
In the above referenced patents and related papers, a metal or metal alloy is first heavily deformed and then annealed at a temperature above its primary recrystallization temperature, but below its secondary recrystallization temperature. This results in the formation of a sharp {100}<100> cube texture. Annealing of the substrate above its secondary recrystallization temperature is avoided since off-cube orientations appear and result in the formation of high angle grain boundaries. As noted above, substrates having high angle grain boundaries are undesirable for most device applications, such as superconducting devices.